The modification of polysaccharides by chemical derivatization is well known in the art. Suitable derivatives include esters, such as acetate, and half esters, such as the succinate and octenyl succinate, prepared by reaction with acetic anhydride, succinic anhydride, and octenyl succinic anhydride, respectively; ethers such as hydroxypropyl ether, prepared by reaction with propylene oxide; phosphate derivatives prepared by reaction with sodium or potassium orthophosphate or sodium or potassium tripolyphosphate; or any other starch derivatives or combinations thereof known in the art.
Techniques used in preparing polysaccharide esters have generally involved aqueous reactions for low degree of substitution esters and non-aqueous systems such as anhydrous pyridine for the high degree of substitution starch esters. Degree of substitution (DS), as used herein, is a measure of the average number of hydroxyl groups on each D-glucopyranosyl unit which are derivatized by substituent groups. Since the majority of D-glucopyranosyl units in starch have three hydroxyl groups available for substitution, the maximum possible DS for starch is 3.0.
A good review of the preparation of starch esters may be found in Starch: Chemistry and Technology, 2.sup.nd Edition, Ed. Whistler, BeMiller, and Paschall, Academic Press, 1984, Chapter X. This reference teaches that low degree of substitution starch acetates can be made by the treatment of an aqueous starch suspension with acetic anhydride at pH 7-11. The maximum degree of substitution obtainable without gelatinization varies with the particular starch, but the upper limit is about 0.5 DS. To reach this DS of 0.5, it is necessary to increase repeatedly the reagent concentrations by filtering the starch from the reaction mixture, resuspending it in 1.25-1.5 parts of water per part of starch, and continuing the acetylation.
Methods in Carbohydrate Chemistry, Vol. IV, Ed. Whistler, Academic Press, 1964, 286-287, discloses that no more than 10.2 g (0.1 mole) of acetic anhydride for 162 g (1.0 mole) of starch, dry basis, should be used when reacting an aqueous slurry of starch with acetic anhydride because the efficiency of the reaction decreases with dilution. If a higher treatment is needed, the dilution effect can be offset by removing the excess water through filtration followed by resuspension of the starch in water. The treatment can be repeated using a second portion of acetic anhydride. Treatment with 0.1 mole of acetic anhydride yields a starch acetate with a DS of about 0.07. By repeated treatment, it is possible to make a starch acetate having a DS of 0.5. For higher degrees of substitution, pyridine is the preferred catalyst/solvent in the non-aqueous acetylation.
Several methods of preparing polysaccharide esters in aqueous systems are known in the art. U.S. Pat. No. 2,461,139 issued Feb. 8, 1949 to Caldwell discloses the reaction of starch with organic acid anhydrides in an aqueous alkaline medium preferably using 0.1 to 5% organic acid anhydride based on the dry starch with quantities higher than 10% being less desirable. The preparation of low DS starch esters in water using magnesium oxide or magnesium hydroxide to control pH is disclosed in U.S. Pat. No. 3,839,320 issued Oct. 1, 1974 to Bauer.
Accordingly, while it is known to prepare starch esters using aqueous systems as described above, such methods have been limited to the preparation of low DS derivatives and even require multiple or repeated treatments because of difficulties that result when using higher amounts of anhydride and alkaline reagents.
Billmers, et al., U.S. Pat. No. 5,321,132, disclose a method of preparing starch esters having an intermediate DS of about 0.5 to 1.8 by using a one step aqueous process with organic acid anhydrides, by reacting starch with high treatment levels of anhydride and high concentrations of alkaline reagent. The anhydride is added over a period of time and the pH is controlled by the addition of alkali during the reaction.
The presence of organic solvent is a disadvantage in the preparation of modified starches. Removal of this solvent requires time and energy and is therefore generally less preferred commercially. Simply leaving the solvent in the dispersion is often unacceptable as an object of preparing starches may be to eliminate organic solvent at the point of product use. Further, reduction of organic solvents is an aim of many businesses from an environmental standpoint.
Wurzberg also discloses the well known reaction of reacting succinic anhydrides with polysaccharides to form the half ester. Such reaction may be carried out in pyridine or, to obtain a high degree of substitution, in glacial acetic acid. There is a limit, however, as to how much succinic ahydride can be reacted with granular starch because at above a 3% treatment level, which results in about a 0.02 DS, the granules swell to such a degree that filtration becomes difficult.
Polysaccharides may also be reacted with substituted cyclic dicarboxylic acid anhydrides of the formula: ##STR1## wherein R represents a dimethyl or trimethyl radical and R' is the substituent group, generally a long hydrocarbon chain. The most important commercial products of this class are the alkenylsuccinnate substituted starches. This reaction is more fully disclosed in U.S. Pat. No. 2,661,349 issued to Caldwell and Wurzburg on Dec. 1, 1953 and U.S. Pat. No. 2,613,206 issued to Caldwell on Oct. 7, 1952.
Techniques for starch phosphoration, by reaction with sodium or potassium orthophosphate or sodium or potassium tripolyphosphate, are well known in the art and described, for example, in Methods in Carbohydrate Chemistry, pp. 294-295. Reaction with sodium tripolyphosphate gives substantially undegraded starch phosphates of a low DS (about 0.02) and with orthophosphate salts gives starch phosphates at a maximum DS of 0.2. Further, pH must be strictly controlled as severe hydrolysis occurs at pH levels below 5.0 and the reaction is inefficient at pH levels above 6.5.
Techniques for etherification of polysaccharides are also well known in the art, particularly the commercially important hydroxypropyl ether prepared by reaction with propylene oxide. A good review of the preparation of starch ethers may also be found in Starch: Chemistry and Technology, Chapter X or Modified Starches: Properties and Uses, Ed. Wurzburg, CRC Press, 1986, Chapters 5 and 6.
The most common method of preparing hydroxyalkyl starch derivatives is by reaction with alkylene oxide under strongly alkaline, aqueous conditions. High alkalinity is necessary for good reaction efficiency. However, it is recommended that such reaction be conducted in a closed vessel under a blanket of nitrogen because of the explosibility of alkylene oxide-air mixtures. A further disadvantage of this process is that salts need to be added to repress swelling and at a DS greater than 0.1, the product becomes difficult to purify.
Granular hydroxyalkyl starch ethers with a DS in the range of 0.75 to 1.0 can be prepared by suspending the alkaline starch in organic solvents such as lower aliphatic alcohols or ketones or higher alcohols. However, the presence of organic solvent is disadvantageous as removal of the solvent requires time and energy and is therefore generally less preferred commercially.
A third method of preparing hydroxyalkyl starch derivatives are the so-called "dry" reactions in which starch is reacted with alkylene oxide under pressure in the presence of a suitable catalyst. Starch moisture levels must be kept above 5% for efficiency and a DS of up to 3.0 may be achieved.
Other polysaccharide modifications known in the art include acid hydrolysis, oxidation, and cross-linking.
Densified, particularly supercritical, fluids offer a desirable alternative to both solvent and aqueous based synthetic methods. Densified fluids, as used herein, refers to fluids which have a density of greater than 0.0001 g/mL at one atmosphere and 0.degree. C. Densified carbon dioxide is of particular interest in that it provides a non-toxic, nonflammable, inexpensive, recyclable and environmentally acceptable solvent.
Carbon dioxide is known to accelerate the absorption of additives into polymers and thus has been used to assist polymer impregnation. Berens, et al., "Application of Compressed Carbon Dioxide in the Incorporation of Additives into Polymers, AlChE Annual Meeting, Washington, D.C. (Nov. 28-Dec. 2, 1988) disclose that the high solubility, diffusivity, and plasticizing action of carbon dioxide makes compressed carbon dioxide useful as a temporary plasticizer to facilitate the absorption of additives into glassy polymers, specifically poly(vinyl chloride), polycarbonate, poly(methyl methacrylate), and poly(vinyl acetate) Sand, U.S. Pat. No. 4,598,006 and Berens, et al., U.S. Pat. No. 4,820,752 disclose impregnating a polymer with an additive under pressure, i.e., at or near supercritical conditions.
Chemical modification of polysaccharides in densified liquids has been limited up to this point. Yalpani, in Carbohydrates and Carbohydrate Polymers, Analysis, Biotechnology, Modification, Antiviral, Biomedical and Other Applications, Yalpani, M. (ed.), ATL Press, 1993, Ch. 23, and Yalpani, "Supercritical Fluids: Puissant Media for the Modification of Polymers and Biopolymers," Polymer vol. 34 (1993) pp. 1102-1105 disclose modification of glycans, such as chitosan, by reaction with glucose or malto-oligosaccharides in an aqueous/supercritical carbon dioxide mixture to produce water soluble, imine-linked branched chitosan derivatives. The water facilitates this reductive alkylation of the imine functions of the chitosan by the carbohydrates which can produce materials of up to 0.8 DS.
Enzymatic hydrolysis of starch to glucose in supercritical carbon dioxide has also been investigated by Lee, et al. See "Starch Hydrolysis Using Enzyme in Supercritical Carbon Dioxide," Biotechnology Techniques, vol. 7, no. 4 (April 1993) pp. 267-270. In this reaction, the enzyme is necessary for the hydrolysis reaction to proceed.
However, the problem of carrying out efficient chemical modifications of polysaccharides in reactions conducted in densified fluids has remained largely unaddressed until the present invention.